Summary
Chapter 4 of the NCERT Class 9 Science textbook (2026-27 'Exploration' edition) covers describing motion, introducing displacement, average speed, average velocity, average acceleration, kinematic equations, and uniform circular motion. It explains how to analyse linear motion using position-time and velocity-time graphs.
Chapter 4 'Describing Motion Around Us' from the NCERT Class 9 Science 'Exploration' textbook (2026-27 edition) explains linear motion and uniform circular motion. Students learn to distinguish between distance and displacement, calculate average speed and average velocity using the formula v_av = s/t, and determine average acceleration as the change in velocity divided by time interval (SI unit: m s⁻²). The chapter introduces three kinematic equations for constant acceleration: v = u + at, s = ut + ½at², and v² = u² + 2as. Motion is also represented through position-time and velocity-time graphs, where slopes give velocity and acceleration respectively, and areas under velocity-time graphs give displacement.
Key points & formulas
- 01Displacement is the net change in position between two instants, requiring both magnitude and direction; it can be zero even when total distance travelled is non-zero (e.g., an athlete who runs and returns to the start)
- 02Average speed equals total distance divided by time interval; average velocity equals displacement divided by time interval (v_av = s/t), with SI unit m s⁻¹
- 03Average acceleration equals change in velocity divided by time interval (a = (v − u)/t), with SI unit m s⁻²; acceleration can exist even when speed is constant, if direction changes
- 04The three kinematic equations for straight-line motion with constant acceleration are: v = u + at, s = ut + ½at², and v² = u² + 2as
- 05On a position-time graph, the slope of the line gives the magnitude of velocity; a straight line indicates constant velocity and a curve indicates changing (accelerated) velocity
- 06In uniform circular motion, an object moves at constant speed in a circular path, but its direction of velocity continuously changes, so it is always accelerating
Frequently asked questions
01What is the difference between distance and displacement in Class 9 Chapter 4?
Distance is the total path length travelled by an object, while displacement is the net change in position between two instants of time. Displacement requires both magnitude and direction. For example, an athlete who runs 100 m forward and 60 m back has travelled 160 m total distance but has a displacement of only 40 m in the forward direction.
02What are the three kinematic equations for uniformly accelerated motion given in Chapter 4?
The three kinematic equations are: (1) v = u + at, (2) s = ut + ½at², and (3) v² = u² + 2as, where u is initial velocity, v is final velocity, a is constant acceleration, t is time interval, and s is displacement. These equations are valid only when acceleration is constant.
03What does the slope of a velocity-time graph represent?
The slope of a straight line on a velocity-time graph represents the average acceleration of the object. If the line is horizontal (zero slope), the velocity is constant and acceleration is zero. A positive slope means the velocity is increasing with constant acceleration, while a negative slope means the velocity is decreasing (deceleration).
04Is the NCERT Class 9 Science Chapter 4 PDF free to download?
Yes, the NCERT Class 9 Science Chapter 4 PDF is free to download on cbseprepmaster.com.
More chapters in Exploration
This is the complete Exploration Chapter 4 as published by NCERT — every diagram, solved example, and exercise included, free. Browse all NCERT Class 9 textbooks.
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